This application claims the priority of PCT International No. PCT/EP099/09704 filed Dec. 9, 1999 and German Patent Document 199 02 444.8, filed Jan. 22, 1999, the disclosures of which are expressly incorporated by reference herein.
The present invention relates to a brake system with hydraulic brake boosting and to a process therefor.
DE 35 26 556 A1 discloses such a brake system. The master brake cylinder is connected to the wheel brake device by means of a brake line by way of a multi-position valve. A servo brake pressure supplying device can be connected by way of the multi-position valve and can feed additional brake pressure to the wheel brake device. A branch line branches off from the brake line between the master brake cylinder and the multi-position valve. Said branch line is connected to the pedal travel simulator and then shut off by means of a solenoid valve, when a fault occurs in the brake system, for example in the servo brake pressure supplying device.
The pedal travel simulator is supposed to achieve a meterable brake pedal behavior that is familiar to the driver and to reach a pedal travel distance that is significant during braking.
When a fault occurs, the pedal travel simulator is completely separated with respect to the hydraulic medium. In this respect the hydraulic medium, located in the pedal travel simulator at the time of the separation by means of the solenoid valve, is locked in and is not available for further brake pressure build-up, for example, in the subsequent braking operations.
The DE 35 26 556 A1 discloses a brake system for motor vehicles, which comprises a master cylinder or a brake pressure generator, which is designed as a tandem cylinder and which is actuated by a brake pedal and drives a wheel brake device with pressurized hydraulic medium, delivered to the wheel brake device from a reservoir. Provided is also an auxiliary pressure supply system that comprises a hydraulic pump and an electric motor, actuating the pump. The medium, conveyed by the pump, can be fed to the wheel brake device via a switching valve.
A pressure chamber of the master cylinder, acted upon by the brake pedal, communicates with the pedal travel simulator, which is designed as a cylinder with a displaceable actuating piston. The actuating piston limits, in the cylinder, a displacement volume, which is connected to the reservoir via a hydraulic line.
When the volume of the pressure chamber of the master cylinder is reduced due to the pressure of the brake pedal, the medium flows out of the pressure chamber into the pedal travel simulator and acts on the actuating piston, which is adjusted in the direction of the displacement volume. In so doing, the medium, contained in the displacement volume, escapes into the reservoir. At the same time the pressure in the displacement volume of the pedal travel simulator is used for the dynamic modulation of the brake pressure, which is generated via the motor-operated hydraulic pump of the auxiliary pressure supply system.
In order to provide sufficient brake force in the event of a failure of the power supply, the brake pressure in this kind of system is not generated, in the event of a fault, by means of the auxiliary supply system, but rather directly by means of the master cylinder. It is hereby possible to prevent hazardous situations, which may arise due to a failure of the hydraulic pump, for example as a result of a breakdown in the power supply or a failure of the control unit of the system.
If in brake systems, described above, a fault occurs during a braking operation, in which the brake pedal is already partially depressed, there is the problem that up until the brake pedal stops only the remaining residual pedal travel is available via the master cylinder for the build-up of brake pressure necessary during emergency situations. This pedal travel is reduced, as compared to the non-actuated brake pedal, and, under some circumstances, is not sufficient for generating the necessary brake force. If the brake pedal is already fully depressed at the moment when the fault occurs, no additional brake pressure can be generated at all by way of the master cylinder.
An object of the present invention is to provide electrohydraulic brakes with simple construction so as to be more fail safe.
This object has been achieved according to the present invention by the brake system and process described herein.
More specifically, a return line is arranged in the hydraulic path between the wheel brake device and the displacement volume of the pedal travel simulator. After the end of the braking operation, said return line conveys the hydraulic medium out of the wheel brake device normally first into the displacement volume and then by means of another hydraulic line back into the reservoir. The hydraulic line between the displacement volume and the reservoir has a return flow valve, which can be adjusted between an opening position, releasing the hydraulic line, and a shut-off position, closing the hydraulic line. Normally the return flow valve is in the open position so that the hydraulic medium flows out of the wheel brake device into the reservoir.
In contrast, in the event of a fault, the return flow valve is shifted into the shut-off position so that, although the connection between the wheel brake device and the displacement volume of the pedal travel simulator is still open, the connection between the displacement volume and the reservoir is interrupted and is consequently hydraulically rigid. The result is that in the event of a fault occurring during a braking operation, the energy stored in the wheel brake device causes the hydraulic medium to flow out of the wheel brake device through the return line back into the displacement volume of the pedal travel simulator. Said displacement volume fills with hydraulic medium and increases its volume as a result of the interrupted connection to the reservoir. The result of the increase in volume of the displacement volume is that the brake pedal, connected to the displacement volume, is set in the direction of its initial position so that the brake pedal travel available to the driver is increased automatically. The additional pedal travel can then be utilized for the brake force, which is generated by the driver and which in the event of faults passes as hydraulic brake pressure through to the wheel brake device directly via the actuating and hydraulic unit.
In an advantageous development, the return-flow valve in the line between the displacement volume and the reservoir is capable of being actuated electrically and, in the dead state, to be in a shut-off position blocking the throughflow. This ensures that, in the event of a fault, the displacement volume is always cut off from the reservoir and therefore the medium conveyed back into the displacement volume must collect in the displacement volume and necessarily leads to an expansion of this volume.